Much recent research has been dedicated to the development of a practical charged-particle-beam (CPB) exposure apparatus exhibiting high resolution and high throughput. One contemporary approach has been directed to batch systems that project at least one entire die in a single exposure (wherein a "die" is coextensive with an integrated circuit or display to be formed on a wafer or other suitable substrate. In many instances, the wafer accommodates multiple dies.) Unfortunately, it is difficult to prepare a "mask" (serving as a projection "master") suitable for use with a CPB batch projection-exposure system. Also, due to the relatively large field of view of the CPB projection-optical system used in such apparatus, it is difficult to maintain aberrations arising in the CPB projection-optical system at or below specifications.
Therefore, another contemporary approach utilizes a segmented mask in which a die is divided into multiple "mask subfields" that are individually projected onto corresponding "transfer subfields" on the sensitized substrate. Each die projected onto the substrate comprises multiple constituent transfer subfields that are "stitched" together. This approach is termed a "divided" projection-exposure apparatus. One advantage of the divided projection-exposure apparatus is that aberrations arising in the projection-optical system tend to be smaller due to the smaller field of view of the projection-optical system. Also, most divided projection-optical apparatus allow projection and exposure to be performed while certain aberrations are being corrected. For example, the focal position for each sub-field and distortion of the projected image can be individually adjusted for each mask subfield. Such aspects of divided projection-exposure apparatus allow exposures to be made with excellent resolution and positional accuracy across an optically wider area than realized with batch projection-exposure apparatus.
The pattern portion defined in each mask subfield is typically demagnified by the projection-optical system by a specified demagnification ratio. I.e., the image of each mask subfield projected onto the corresponding transfer subfield is reduced in size ("demagnified"), usually by an integer factor (e.g., 2, 4, or 5). Although the demagnification ratio is typically fixed at a nominal value, the demagnification ratio can change slightly over time as influenced by, e.g., changes in environmental conditions.
Changes in environmental conditions and/or prolonged use of a CPB exposure apparatus can also cause changes in aberrations exhibited by the projection-optical system. For example, an image formed on a transfer subfield can exhibit some degree of rotation relative to the orientation of the corresponding mask subfield. Also, loading a mask or wafer can introduce a rotational error or other alignment error of the pattern to be transferred, relative to the previously transferred patterns of existing layer(s) on the wafer. Such rotation errors can cause substantial difficulty in achieving satisfactory stitching together of the transfer subfields on the substrate and in achieving accurate registration of the various layers with each other.
Certain CPB projection-exposure apparatus have been proposed (e.g., in Japan Kokai Patent Publication No. HEI 7-22349, and Japan Kokai Patent Publication No. HEI 8-132987) that measure errors in the demagnification ratio and/or rotation of a projected mask-subfield image, and controllably reduce such errors. In such apparatus, the "seams" between adjacent transfer subfields of the same layer on the substrate can be accurately aligned. However, it is difficult with such apparatus to achieve a satisfactorily accurate alignment of features of a layer (e.g., a second layer) with features in an earlier-applied layer (e.g., a first layer) on the substrate (termed "overlay errors"). Whereas overlay errors do not pose serious problems when 5-.mu.m.sup.2 subfields are projected onto the substrate, larger subfields (e.g., 250-.mu.m.sup.2 subfields) can be problematic with respect to achieving a desired registration accuracy between, e.g., the second-layer pattern and the first-layer pattern.